Using carbonate sediments and fossil shells to assess how the composition of seawater has changed is a long-standing technique in sedimentary geochemistry. Isotopes of strontium and oxygen have provided revolutionising windows on the pace of continental weathering and fluctuations in sea-surface temperature and continental ice cover for over 30 years. The magnesium to calcium ratio in fossil shells has given insights into deep-water temperatures for the Cenozoic, more recently. However, tracking changes in the bulk composition of seawater through time, through analyses of carbonates, is plagued by the continual chemical interaction between rocks and the waters with which they are in contact. The Mg/Ca ratio of sea water is a potential proxy for the amount of hydrothermal activity on the sea floor, and thus the rate of sea-floor spreading. This is not because oceanic basalts are magnesium rich compared with continental crust that provides much of the dissolved matter that enters the oceans, but because hydrothermal reactions tend to mop up dissolved magnesium and release calcium.. Unfortunately, magnesium also easily replaces calcium in carbonates during diagenetic processes, particularly dolomitisation. There are two means of overcoming this hindrance, by analysing seawater trapped as fluid inclusions in evaporite minerals and the shells of echinoderms that still contain minute structures formed in life and are unlikely to have been altered (Dickson, J.A.D. 2002. Fossil echinoderms as monitor of the Mg/Ca ratio of Phanerozoic oceans. Science, v. 298, p. 1222-1224). Early results seem to match a prediction that while supercontinents existed, the length of mid-ocean ridges and therefore ocean floor hydrothermal activity were at a minimum. Around the Precambrian boundary and during the Carboniferous to Jurassic periods, Mg/Ca was high at the time of the Vendian and Pangaea supercontinents. During major bouts of continental break up – the Lower Palaeozoic and Mesozoic – the ratio is low. Oddly, the ratio has risen to unprecedented high levels during the Cenozoic Era, when clearly there is high hydrothermal activity.
Despite the fact that the Mg-Ca record of the oceans is limited to just a few short time spans in the 545 Ma record of the Phanerozoic, plenty of geochemists and palaeobiologists are speculating about the possible consequences for evolution of changes in the bulk composition of seawater. There have been major swings in the proportion of calcite to dolomite in carbonate sediments throughout geological time (see Bacteria and dolomites, January 2001 Earth Pages News). Discussion now centres on the possible effect of changing Mg/Ca ratios on the waxing and waning of important carbonate secreting organisms, ranging from corals and molluscs that build reefs to the minute coccoliths that formed the Cretaceous Chalk. Perhaps different groups responded differently to changing water composition, and maybe the Cambrian Explosion of shelly faunas was triggered somehow by a critical shift in the ratio.
See also: Kerr, R.A. 2002. Inconstant ancient seas and life’s path. Science, v. 298, p. 1165-1166
Deep carbon cycling, and gold mineralization
One of the more speculative aspects of the carbon cycle concerns the fate of carbonate sediments that descend subduction zones. One popular hypothesis, with an acronym that is likely to amuse colloquially inclined, British readers (the BLAG model named after its three originators Berner, Lasaga and Garrels) avows that such carbonates contribute to CO2 emissions from volcanoes above subduction zones by reacting with silica. The presence in blueschists of abundant aragonite associated with silica suggests that if that does happen, not all carbonate is consumed and a great deal enters very long-term storage in the mantle. Indeed, aragonite-magnesite associations are stable to pressures that are equivalent to depths of 240 km. Rocks formed under exceptionally high-pressure conditions, which might shed further light on the deep part of the carbon cycle, are exceptionally rare. One such occurrence is the Kokchetav massif of Kazakhstan, in which dolomitic marbles accompany eclogites. Notable for the occurrence of metamorphic diamonds, Kokchetav rocks probably equilibrated deeper than 250 km, so the carbonates are particularly interesting. Yongfeng Zhu and Yoshihide Ogasawara of Beijing University in China and Waseda University in Japan have found evidence for dissociation of dolomite in them (Zhu, Y. & Ogasawara, Y. 2002. Carbon recycled into deep Earth: Evidence from dolomite dissociation in subduction-zone rocks. Geology, v. 30, p. 947-950) during reactions that generate garnet and clinochlore. The mineral textures reveal equilibria that involve the production of carbon and oxygen, rather than CO2, so it is quite possible that reflux of CO2 from subduction zones to the atmosphere may not be as significant as the “BLAGgers” suppose.
Interestingly, the same issue of Geology includes a paper on the geochemical conditions under which gold and copper enter subduction-zone magmas to source major ore deposits (Mungall, J.E. 2002. Roasting the mantle: Slab melting and the genesis of major Au and Au-rich Cu deposits. Geology, v. 30, p.915-918). Mungall focuses on the inability of chalcophile metals to enter magmas when sulphides are stable in the mantle. Under those condition Au and Cu tend to enter sulphide melts whose density and immiscibility separate them from silicate melts. Oxidation of sulphur is needed to overcome this tendency, and that requires high oxygen fugacity at the depths involved, suggested by him to accompany abundant iron-3 in the subducted materials. That may be so, but release of molecular oxygen by high-pressure carbonate dissociation, as described by Zhu and Ogasawara, seems an even more likely means of freeing chalcophile metals to magmas.